Reticle cleaning without damaging pellicle

ABSTRACT

A reticle having a pellicle frame and pellicle membrane is cleaned without removing or damaging the pellicle membrane. A cover encases the pellicle membrane and pellicle frame, sealing the pellicle from the external environment during a cleaning process. The cover fits around the periphery of the pellicle frame and covers the pellicle membrane. An edge of the cover in contact with the reticle forms a seal. The reticle is fastened to reticle supports on a spin chuck during the cleaning process. An anchor plate presses the cover to the reticle, maintaining the pellicle sealed from the external environment. The cover and reticle are sandwiched together between the anchor plate and spin chuck.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 08/921,656,filed Aug. 28, 1997, now U.S. Pat. No. 5,938,860, issued Aug. 17, 1999.

BACKGROUND OF THE INVENTION

This invention relates to integrated circuit fabrication tools andprocesses and, more particularly, to a method and apparatus for cleaninga pellicled reticle.

Integrated circuits (IC) commonly are fabricated on a semiconductorwafer. The semiconductor wafer typically is subjected to doping,deposition, etching, planarizing and lithographic processes to formsemiconductor devices in the wafer. The wafer typically is cut to formmultiple semiconductor "IC chips". Each chip includes many semiconductordevices. Although the label semiconductor is used, the devices arefabricated from various materials, including electrical conductors(e.g., aluminum, tungsten), electrical semiconductors (e.g., silicon)and electrical non-conductors (e.g., silicon dioxide).

A reticle is used in a lithographic process to define a photomask. Alithographic process refers to a process in which a pattern isdelineated in a layer of material (e.g., photoresist) sensitive tophotons, electrons or ions. The principle is similar to that of aphotocamera in which an object is imaged on a photo-sensitive emulsionfilm. While with a photo-camera the "final product" is the printedimage, the image in the semiconductor process context typically is anintermediate pattern which defines regions where material is depositedor removed. The lithographic process typically involves multipleexposing and developing steps, wherein at a given step the photoresistis exposed to photons, electrons or ions, then developed to remove oneof either the exposed or unexposed portions of photoresist. Complexpatterns typically require multiple exposure and development steps.

A typical lithographic system includes a light source, optical systemand transparent photomask. The light source emits light through theoptical system and photomask onto a photoresist layer of a semiconductorwafer. The photomask defines the "intermediate pattern" used fordetermining where photoresist is to be removed or left in place.Conventional photomasks are transparent masks. A photomask typically isformed on a glass blank. The mask and blank together are referred to asa reticle. Conventional materials for the blank include soda lime,borosilicate glass or fused silica. The photomask is formed by a film ofopaque material. Typically, the film is formed with chrome less than 100nm thick and covered with an anti-reflective coating such as chromeoxide. The purpose of the anti-reflective coating is to suppress ghostimages from the light reflected by the opaque material.

The photomask serves to define geometries for materials deposited oretched on the wafer or materials applied to the wafer. The patternedfilm on the reticle blank includes mask lines and line spacings of lessthan 10 microns. Depending on the reduction factor x, line width andline space geometries for a resulting semiconductor device are from lessthan 10 microns to less than 2 microns. Other mask line spacings andsemiconductor line spacings also can be achieved. When working with suchsmall geometries, it is important that the reticle and other componentsin the fabrication processes be free of foreign particles. A tiny speckof dust alters the desired pattern to be imaged onto the wafer.Conventionally, a thin transparent membrane, referred to as a pelliclemembrane, is applied over the photomask portion of the reticle to keepthe photomask portion free of foreign particles. The pellicle membranetypically is positioned at a height above the photomask. Such height isgreater than the focal length of the light imaged onto the photomask.Thus, small particles on the pellicle membrane will not block light fromreaching the photomask.

Another problem caused by foreign particles is bad registration of thereticle. During a lithographic process, the reticle rests on a reticletable. The reticle table typically is part of a stepper device whichalso includes a light source and a stepper control. The stepper controlmanages the relative position of the light source and the reticle table.Even the smallest of particles on the edge of the reticle can lift aportion of the reticle off the reticle table. Such offset of the reticlecan result in bad registration of the light onto the wafer, which inturn can result in bad overlay from one pattern to another. Because thepellicle membrane typically is very fragile, the pellicle membrane isdestroyed during the course of cleaning the reticle. Conventionally, thepellicle membrane is removed, the entire surface of the reticle iscleaned, and then the reticle undergoes requalification. Such a processis very time consuming and costly. Accordingly, there is a need for analternative method and apparatus for cleaning a reticle.

SUMMARY OF THE INVENTION

According to the invention, a reticle having a pellicle is cleanedwithout removing or damaging the pellicle. The pellicle includes apellicle membrane and a pellicle frame. A cover encases the pellicle,sealing it from the external environment during the cleaning process.

According to one aspect of the invention, the cover fits around theperiphery of the pellicle frame and covers the pellicle membrane. Anedge of the cover in contact with the reticle (adjacent the pellicleframe) forms a seal. In a preferred embodiment, the edge includes agroove within which is an O-ring seal.

According to another aspect of the invention, the reticle is fastened toreticle supports on a spin chuck during the cleaning process. An anchorplate presses the cover to the reticle, maintaining the pellicle sealedfrom the external environment. The anchor plate fastens to the spinchuck. Thus, the cover and reticle are sandwiched together between theanchor plate and spin chuck.

According to another aspect of the invention, a system for cleaning areticle having a pellicle frame and pellicle membrane is provided. Thereticle has a pattern formed on a first surface, the pattern occurringwithin a first area of the first surface. The pellicle frame is attachedto the first surface, defining a border of the pattern. The pelliclemembrane is attached to the pellicle frame and elevated above thepattern, the pellicle membrane sealing the first area. The reticle issecured to a support. A lid encases the pellicle frame and pelliclemembrane. The lid has a first surface in contact with the first surfaceof the reticle. A clamp pushes the lid to the reticle.

According to another aspect of the invention, the lid has a firstsurface in contact with the first surface of the reticle. The lid has agroove formed within the lid's first surface. An O-ring seal is withinthe groove. The O-ring seal is pressed into contact with the reticle bythe lid under a force of the clamp. The clamp is secured to the support.

According to another aspect of the invention, a fluid under pressure isejected onto the reticle, wherein the pellicle membrane is shielded fromthe fluid by the lid. A drive mechanism rotates the support, altering aportion of the reticle receiving the fluid under pressure.

According to another aspect of the invention, the lid and reticle serveas an apparatus for encasing the pellicle. The lid has a recessed areawhich is bordered peripherally by a first wall. The first wall isadjacent to a first edge. The first edge has a seal extending around aperipheral border of the recessed area. The first wall has a heightgreater than a height of the pellicle frame. The lid encases thepellicle membrane and pellicle frame within the recessed area with theseal making contact with the reticle on the first surface.

According to another aspect of the invention, the seal is an O-ring sealwithin a groove along the first edge for sealing the recessed area ofthe lid and the enclosed pellicle frame and pellicle membrane from anenvironment of the reticle.

According to another aspect of the invention, a method for cleaning areticle without damaging or removing a pellicle membrane is performed.At one step, the pellicle is covered with a lid to separate the pelliclefrom an external environment of an uncovered portion of the reticle. Atanother step, a force is applied to the lid to seal the pellicle fromthe external environment. At another step, the reticle is secured to abase. At another step, fluid under pressure is ejected onto theuncovered portion of the reticle to clean the uncovered portion offoreign particles.

An advantage of the invention is that a reticle which does notaccurately rest on a stepper table due to foreign particles is cleanedwithout removing or damaging the pellicle. An effect of this advantageis that the reticle does not need to go through an extensive process ofre-applying a pellicle frame and pellicle membrane and requalifying thereticle for use in a lithographic process. These and other aspects andadvantages of the invention will be better understood by reference tothe following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional lithographic system withphotomask and wafer;

FIG. 2 is a perspective view of a conventional reticle with pellicleframe and pellicle membrane;

FIG. 3 is a side view of the reticle, pellicle frame and pelliclemembrane of FIG. 2;

FIG. 4 is a diagram of a reticle cleaning system according to anembodiment of this invention;

FIG. 5 is a plan view of a spin chuck of FIG. 4 according to anembodiment of this invention;

FIG. 6 is a plan view of a pellicle cover of FIG. 4 according to anembodiment of this invention;

FIG. 7 is a side view of the pellicle cover of FIG. 6, along with a sideview of an O-ring; and

FIG. 8 is a perspective view of an anchor cap of FIG. 4 according to anembodiment of this invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Overview

FIG. 1 shows a block diagram of a conventional lithographic system 10.The lithographic system 10 includes a light source 12 which emits light13 (e.g., ultraviolet light, visible light, infrared light). The lightpasses through a mask formed on a reticle 14, then through an opening ina reticle table 16, and onto a semiconductor wafer 18. A steppercontroller 20 (also known as an aligner) controls the relativepositioning of the light source 12 and reticle table 16. Typically, thelight 13 serves to develop portions of photoresist applied to the wafer.The mask defines a pattern distinguishing which portions of thephotoresist are developed and which are not developed.

FIGS. 2 and 3 show a reticle 14. The reticle includes a transparentplate 22 or "blank" covered with a patterned film 24 of opaque material(i.e., the photomask). Although the size may vary, an exemplary reticle14 is 6 inches by 6 inches and 0.25 inches thick. Conventional materialsfor the blank include soda lime, borosilicate glass or fused silica. Thefilm of opaque material typically is a film of chrome less than 100 nmthick and covered with an anti-reflective coating such as chrome oxide.Within an area 26, the film 24 defines masks 28 for respective portionsof the wafer 18. For example, in one embodiment illustrated, fifteenmasks are shown. Each mask 28 within the area 26 may be the same ordifferent, so as to make the same or different integrated circuits.

Attached to the reticle is a pellicle frame 30. In an exemplaryembodiment, the pellicle frame 30 is adhered to the reticle 14 by doubleback tape. Other adhesive structures may be used, however. The pellicleframe 30 encloses the area 26 of the reticle having the masks 28.Adhered to the pellicle frame 30 is a thin membrane, referred to as apellicle membrane 32. The pellicle membrane seals the area 26 from theexternal environment. As described in the background section, it isdesirable to avoid foreign particles on a photomask. When a reticle withmasks 28 is formed, the surface is cleaned and qualified to assure thatthe mask is accurate and that no foreign particles are present. As partof such qualification process, the pellicle membrane 32 is adhered tothe pellicle frame 30. The pellicle membrane 32 protects the masks 28from foreign particles. The pellicle membrane is formed of aconventional material such as cellulose acetate or nitrous cellulose.

As shown in FIG. 1, the reticle 14 rests on a reticle table 16 duringthe lithographic process. The lithographic processes often require thata given reticle 14 be replaced from the reticle table 16 with anotherreticle having a different mask pattern. This movement of reticles onand off the reticle table 16 can cause microscopic particles to adhereto the reticle 14. Further, reticles typically are stored in a carryingcase. Microscopic particles also may adhere to the reticle from rubbingalong rails of the reticle carrying case. If there are any foreignparticles on the reticle in the regions 33 (see FIG. 2) where thereticle 14 is supposed to contact the reticle table 16, then the reticlemay not be seated exactly. A portion of the reticle may be higher thananother portion. This can result in bad registration of the lightpassing through a mask 28 onto a wafer, or to bad overlay from one maskto another mask. If such problem is detected, the reticle is removed andcleaned. Because the pellicle membrane 32 typically is very fragile, thepellicle membrane is destroyed during the course of cleaning thereticle. Conventionally, the pellicle is removed and the entire surfaceof the reticle is cleaned. The pellicle frame and a pellicle membranethen are reapplied, and the structures 14, 30, 32 requalified for thedesired lithographic operations.

FIG. 4 shows a cleaning system 40 according to an embodiment of thisinvention. During cleaning, the pellicle membrane 32 and pellicle frame30 are covered to avoid damage. A lid 42 encases the pellicle membrane32 and pellicle frame 30, sealing the pellicle from the externalenvironment of the cleaning system. In one embodiment, an O-ring 44defines the seal between the lid 42 and the reticle 14 adjacent to thepellicle frame 30. The O-ring 44 or seal is formed from silicon oranother material.

During a cleaning operation, the reticle 14 is secured to a spin chuck46. The spin chuck 46 includes reticle supports 48. The reticle 14 restson the supports 48. An anchor plate 50 resides on top of the lid 42. Theanchor plate 50 is bolted to the spin chuck 46, pressing the lid 42 tothe reticle 14 to maintain the seal, and pressing the reticle to thereticle supports 48. Thus, the reticle 14 and lid 42 are sandwichedbetween the anchor plate 50 and spin chuck 46.

With the lid 42 and reticle 14 secure, a rotary drive 52 rotates thespin chuck 46. In addition, a spray source 54 ejects a fluid to cleanand rinse the reticle 14. In one embodiment, de-ionized water or anotherfluid is ejected as a fan spray 57 to the upper surface of the anchorplate 50, reticle 14 and spin chuck 46 assembly and as a rinse spray 59to a lower surface of such assembly. Then a fluid under pressure (e.g.,500 psi) is ejected as a high pressure spray 56 onto at least theexposed portions of the reticle 14 to clean away any foreign particleson the reticle 14. The fluid ejected from the fan spray and rinse sprayis de-ionized water in one embodiment, although other liquid or gasfluids may be used. The fluid ejected from the high pressure spray 56 isammonium hydroxide, de-ionized water and/or another liquid or gas fluid.In one method for cleaning the reticle, the spin chuck 46 rotates at1500 revolutions per minute during the ejection of the fluids. The highpressure spray 56 then ceases, followed by cessation of the fan spray 57and rinse spray 59. The spin chuck 46 then increases the rotational rate(e.g., to 2000 rpm) during a drying time period. The speeds ofrevolution, the pressure of the fluids emitted from sprays 56, 57 and 59and the time for spraying and drying the assembly may vary.

The reticle 14, being secured to the spin chuck 46, rotates with thespin chuck 46. Rotation of the reticle 14 places different exposedportions of the reticle 14 in the path of the high pressure fluid spray56. In a preferred embodiment, the portion of the reticle 14 which is incontact with the reticle table 16 during a lithographic process isexposed during the cleaning process. Specifically, such portion is notcovered by the lid 42.

FIG. 5 shows a spin chuck 46 according to an embodiment of thisinvention. The spin chuck 46 serves as a base to which the othercomponents are secured. The base 46, either with or without the reticlesupports 48, serves as a support for the reticle 14 (e.g., in oneembodiment, supports are integral to the base). In one embodiment, thespin chuck 46 is of sufficient area that a portion of the spin chuck 46is exposed when the reticle 14 is secured to the spin chuck. Openings 58occur in the exposed areas along opposite edges 60, 62 of the reticle14. Such openings receive pins 64 (see FIG. 8) which secure the anchorplate 50 to the spin chuck 46.

In various embodiments, the spin chuck 46 has different shapes (e.g.,circular, square, rectangular, or other shape). In the embodimentillustrated, the spin chuck is a ring 66 with spokes 68 extending from acentral portion 70. Multiple reticle supports 48 are attached to thespin chuck 46. In one embodiment, the reticle supports 48 are bolted tothe spin chuck 46. In another embodiment, the supports 48 are integralto the rest of the spin chuck 46. Each reticle support has a distalsurface or pin 72 upon which the reticle 14 rests during cleaning. Thespin chuck 46 is rotated by the rotary drive 52.

FIGS. 6 and 7 show the lid 42 for covering the pellicle frame 30 andpellicle membrane 32. The lid is generally planar, defining two faces73, 76. One face 76 defines a generally planar exterior surface. Thecontour of the exterior surface 76, however, need not be planar and mayvary. The other face 73 defines a distal surface 80 and a recessed area74. The recessed area is delimited by an interior surface 82 and a wall84 and a distal surface 80. The wall 84 extends from the interiorsurface 82 to the distal surface 80. When the lid 42 is applied over thepellicle onto the reticle 14, the pellicle frame 30 and pelliclemembrane 32 are enclosed within the recessed area 74. Accordingly, theheight of the wall 84 relative to the interior surface 82 is greaterthan a height of the pellicle frame 30. The lid 42 includes a seal alongthe distal surface 80. In one embodiment, the seal is formed by a groove86 and an O-ring 44. In an exemplary embodiment, the groove is 0.07inches wide with a depth of 0.04 inches. The distal surface 80 spans awidth of 0.2 inches. Such dimensions, however, vary for differingembodiments. The O-ring 44 seats within the groove 86 and extends alongthe entire circumference of the distal wall 80 so as to form a seal allthe way around the pellicle frame 30. In other embodiments, analternative sealing device is used, such as a gasket. Preferably, theseal and lid 42 are formed of material which does not readily chip. Theadvantage of such material is the avoidance of leaving foreign particleson the reticle 14 when the lid 42 is removed from the reticle 14. Whenthe lid 42 is applied to the reticle 14, the pellicle frame 30 andpellicle membrane 32 are completely encased between the lid 42 andreticle 14. When the lid 42 is pressed to the reticle 14, the sealisolates the pellicle membrane 32 from the environment of the cleaningsystem 40, and in particular from the ejected fluid. As the ejectedfluid would break the pellicle membrane 32, the lid 42 prevents thepellicle membrane 32 from being damaged during the cleaning process.

FIG. 8 shows the anchor plate 50 which clamps the lid 42 to the reticle14 and holds the reticle 14 to the spin chuck 46. The anchor plate 50includes a recessed area 90 bordered by two opposing walls 92, 94. Inthe embodiment illustrated, the recessed area 90 is not enclosed. Theanchor plate 50 fits over the lid 42 with the lid 42 fitting between thewalls 92, 94 of the recessed area 90. In one embodiment, the lid 42 ismounted to the anchor plate 50 with screws. The walls 92, 94 fix theorientation of the lid 42 relative to the reticle 14, so as to preventmovement, displacement or offset of the lid 42 by the ejected fluidduring cleaning. The anchor plate 50 defines openings 96 which receivethe pins 64. The pins 64 pass through the openings 58 in the spin chuck46 (see FIG. 5). In one embodiment, a respective screw 65 extends into athreaded opening of each pin 64. The screw 65 pushes the anchor plate 50toward the spin chuck 46. In alternative embodiments, the pins areintegral to either the anchor plate 50 or spin chuck 46. In anotherembodiment, an alternative clamp (e.g., C-clamp; nut and bolt) is usedto secure the anchor plate 50 to the spin chuck 46.

Meritorious and Advantageous Effects

An advantage of the invention is that a reticle which does notaccurately rest on a stepper table due to foreign particles is cleanedwithout removing or damaging the pellicle membrane. An effect of thisadvantage is that the reticle does not need to go through an extensiveprocess of re-applying a pellicle frame and pellicle membrane andrequalifying the reticle for use in a lithographic process. Although apreferred embodiment of the invention has been illustrated anddescribed, various alternatives, modifications and equivalents may beused. Therefore, the foregoing description should not be taken aslimiting the scope of the inventions which are defined by the appendedclaims.

What is claimed is:
 1. A system for cleaning a reticle without damagingor removing a pellicle membrane of the reticle, the system comprising:asupport, the reticle disposed on said support and having a pattern on afirst surface thereof; the pellicle membrane disposed over said patternand spaced apart therefrom by a pellicle frame; a lid disposable incontact with said first surface so as to enclose and seal the pelliclemembrane; a securing member configured to bias said lid against saidfirst surface; and a fluid source from which fluid can be directed ontoan area of the reticle not covered and sealed by said lid.
 2. The systemof claim 1, wherein a region of a second surface of the reticle oppositesaid pattern is exposed.
 3. The system of claim 1, further comprising arotation component associated with said support, which is configured tochange a portion of said reticle exposed to said fluid source.
 4. Thesystem of claim 1, wherein said lid includes a sealing member disposedadjacent a peripheral edge thereof.
 5. The system of claim 4, whereinsaid sealing member is disposed at least partially within a groovedefined in said peripheral edge.
 6. The system of claim 5, wherein saidsealing member protrudes from said peripheral edge.
 7. The system ofclaim 4, wherein said sealing member comprises an O-ring.
 8. The systemof claim 4, wherein said sealing member is configured to contact saidreticle upon disposal of said lid over said pellicle membrane.
 9. Thesystem of claim 1, wherein said support comprises a base and a pluralityof elevated edges configured to contact said reticle.
 10. The system ofclaim 1, wherein said securing member is securable to said support. 11.The system of claim 1, wherein said fluid securing member comprises aclamp.
 12. The system of claim 1, wherein said source is configured tospray fluid.
 13. A method of cleaning a reticle, comprising:shielding apellicle membrane disposed over a patterned region of the reticle from acleaning medium by covering and sealing said pellicle membrane with alid; and directing a cleaning medium into contact with an unshieldedregion of the reticle.
 14. The method of claim 13, wherein saidshielding comprises disposing said lid over said patterned region. 15.The method of claim 14, wherein said shielding comprises securing saidlid over said patterned region so as to seal said lid against thereticle.
 16. The method of claim 13, further comprising directing saidcleaning medium toward another unshielded region of the reticle.
 17. Themethod of claim 16, wherein said directing said cleaning medium towardanother unshielded region of the reticle comprises rotating the reticle.18. A method of cleaning a reticle without damaging or removing apellicle membrane disposed over a first surface of the reticle,comprising:disposing a lid over the pellicle membrane to cover and sealsaid pellicle membrane; and directing a quantity of fluid into contactwith a region of the reticle not shielded by said lid.
 19. The method ofclaim 18, further comprising biasing said lid against said firstsurface.
 20. The method of claim 18, wherein said directing saidquantity of fluid comprises directing pressurized fluid.
 21. The methodof claim 18, wherein said directing said quantity of fluid comprisesremoving foreign particles from said region of the reticle not shieldedby said lid.
 22. The method of claim 18, wherein said directing saidquantity of fluid comprises directing said quantity of fluid toward asecond surface of the reticle, opposite the first surface.
 23. Themethod of claim 18, further comprising rotating at least one of thereticle and a fluid source relative to one another.